What Causes Irregular Menstruation With Arm Insert Birth Control?

Hello, I had my period late this month, but everything went fine when I stared. I have the arm insert birthcontrol . I had it for about 5-6 days which is normal for me. Two days later I had intercorse and the next day I was bleeding again. It was thin bright red blood. Right off the bat I didn't think something was right. I got cramping. And two days later it stopped. I have never had irregular periods.